1. Field of the Invention
The invention relates to a friction clutch for motor vehicles having a clutch casing which is connectable in a rotationally fixed manner to a driven flywheel, a pressure plate arranged inside the clutch casing, and a pressure spring which is supported on the clutch casing for exerting a varying pressure force on the pressure plate during engagement of the clutch to press a clutch disk with friction linings onto the flywheel.
2. Description of the Related Art
A prior art friction clutch is known, for example, from German reference DE 39 40 917 A1. This reference discloses a friction clutch having a clutch casing in which a pressure plate is rotatably fixed and axially movable. A pressure spring is actuatable for pressing the pressure plate relative to the clutch casing onto a clutch disk. An actuation device which acts on the pressure spring and is mechanically or hydraulically actuated is provided to engage and disengage the clutch. The actuation device is arranged partly inside the clutch casing and the actuating member is guided out of the clutch casing. The friction clutch transmits the torque via the clutch disk which is connected to a transmission input shaft. Both sides of the clutch disk have friction linings arranged on a radially outer area. The clutch disk is pressed onto the flywheel of an internal combustion engine by the pressure plate. The pressure spring is designed as a diaphragm spring, and the actuating force which it applies determines the amount of torque which can be transmitted. If the pressure spring is lifted off the pressure plate, the pressure plate is released from the clutch disk and the torque which can be transmitted is reduced, possibly to zero. If the internal combustion engine fails to run smoothly--a characteristic of the internal combustion engines which in design terms is impossible to rule out--the resulting torque fluctuations and torque peaks, which would otherwise lead to increased wear, have to be transmitted without the clutch slipping. A similar problem arises during acceleration of a high-torque internal combustion engine or when a vehicle is moved off in a high gear. As a consequence of these factors, the pressure force which the diaphragm spring has to apply to the pressure plate must be correspondingly high, a fact which in principle requires the clutch to have excessive dimensions.
Since the space available in a motor vehicle--and therefore the available clutch friction radius--are limited, it is necessary to achieve a higher clutch moment by a higher pressure force, resulting in higher actuating forces. To avoid a reduction in comfort for the driver of the vehicle as a result of the high disengagement force required, servo devices are fitted, which boost the actuating force of the clutch pedal.
Inorganic friction linings which have a high coefficient of friction .mu. (for example greater than or equal to 0.3) may also be used to increase the torque which can be transmitted and therefore to reduce the pressure force required. However, the use of such friction linings has the undesirable side effect that the coefficient of friction .mu. of these friction linings, within a certain range, falls as the relative speed increases. Such friction linings may cause clutch grabbing when the clutch is engaged, which represents a considerable reduction in comfort for the passengers of the vehicle.